A hydraulic system may include a variety of components, including multiple hydraulic loads, each of which may have different flow and pressure requirements that can vary over time. The hydraulic system may include a pump for supplying a flow of pressurized fluid to the hydraulic loads. The hydraulic system may further include various valves for controlling the distribution of pressurized fluid to the multiple loads. The valves may include a variety of configurations to accommodate various performance, packaging, and cost requirements of a particular application. For example, applications that have varying hydraulic flow and pressure requirements may employ a proportional valve. Proportional valves have the ability to selectively vary the rate at which hydraulic fluid passes through the valve, and its corresponding pressure. Proportional valves may be electronically or hydraulically actuated. Electronically actuated valves may, for example, employ a proportional solenoid to actuate the valve. The flow of hydraulic fluid through the solenoid actuated proportional valve is generally proportional to the magnitude of an electrical input command signal sent to the solenoid.
A hydraulically actuated proportional valve may also be actuated using a pilot stage. The pilot stage operates to control a flow of pressurized hydraulic fluid for actuating a main stage of the proportional valve. The pilot stage may be electrically of pneumatically operated. An electrically actuated pilot stage, for example, may utilize a high speed solenoid valve to actuate the pilot stage.
Electronically actuated proportional valves and hydraulically actuated proportional valves each have advantages and disadvantages. For example, electronically actuated proportional valves generally allow more precise control of hydraulic flow through the valve than may be achievable with hydraulically actuated proportional valves. In contrast, hydraulically actuated proportional valves are generally capable of controlling large volumes of fluid at high pressures, whereas a solenoid actuated proportional valve, used in the same capacity, would generally be much larger and require large amounts of electricity with unacceptable heat generation. Hydraulically actuated proportional valves generally are capable of faster response times than correspondingly sized electronically actuated proportional valves.